1. Field of the Invention
This invention relates to a piezoelectric thin film resonator and devices provided with the same, and more particularly relates to a piezoelectric thin film resonator which operates on longitudinal vibrations produced along a thickness of a piezoelectric thin film, and a high frequency filter and a voltage-controlled oscillator provided with such piezoelectric thin film resonators.
2. Description of the Related Art
In order to meet development and technological innovation of radio communications, communication devices compatible with a plurality of transmission-reception systems are in much demand. Sophisticated mobile radio terminals are required to be constituted by miniaturized components, modularized components or the like. Filters occupy a large part of passive components in a mobile radio terminal circuit. Therefore, the filters should be miniaturized or modularized in order to miniaturize the circuit and reduce the number of components therein.
Dielectric filters, surface acoustic wave (SAW) filters, LC filters or the like are very popular at present. Filters constituted by thin film bulk elastic wave resonators (piezoelectric thin film resonators) seem most promising in view of the miniaturization and modularization. Such filters operate on resonances of piezoelectric layers. Even when the filters are closely placed, they do not interfere with one another. Closely placed filters may interfere with one another when they operate on electromagnetic waves. Therefore, the filters constituted by piezoelectric thin film resonators can be easily miniaturized compared with dielectric filters or LC filters.
Since high frequency bands are used for the radio communications, SAW filters which operate on surface waves have to be micro-fabricated on a sub-micron level, and cannot be manufactured at a moderate price. On the contrary, filters constituted by piezoelectric thin film resonators operate on longitudinal vibrations produced along a thickness of a piezoelectric layer. An operation band of the filter can be easily raised by thinning the piezoelectric layer. Since a planar size of the filter is required to be approximately 1 μm at most, a fabricating cost of the filter will not be increased even when the operation frequency band is raised. Further, piezoelectric thin film resonators are not made of piezoelectric substrates which are in heavy usage for SAW filters, but are made of semiconductor substrates such as Si substrates or GaAs substrates. LSI chips and filters can be monolithic.
With the piezoelectric thin film resonator (called the “resonator”), energy of excited elastic vibrations should be trapped, so that a top and a bottom of the resonator face with a cavity. An acoustic impedance of a piezoelectric layer of the resonator and an acoustic impedance of an electrode applying a voltage to the piezoelectric layer differ from an acoustic impedance of the air by approximately several figures. Therefore, elastic vibrations are effectively reflected at a border between the electrode and air, which enables the energy of elastic waves to be trapped in the resonator. The top of the resonator is exposed to the air. It is technically important how to make a cavity at the lower part of the resonator.
Japanese Patent Laid-Open Publication No. 2000-069,594 (called the “Reference 1”) discloses a method of manufacturing a piezoelectric thin film resonator. In the resonator, a sacrificial layer is embedded in a substrate surface, and a top electrode, a piezoelectric layer and a bottom electrode are formed on the substrate surface. The sacrificial layer is finally removed, thereby making a cavity. Further, Japanese Patent Laid-Open Publication No. 2001-185,985 (called the “Reference 2”) discloses a method of manufacturing a piezoelectric thin film resonator, in which a resonator is formed on a substrate surface, and the substrate is etched away on a back side thereof in order to make a cavity. In the References 1 and 2, the resonators in which the bottom electrodes, piezoelectric layers and top electrodes are stacked are suspended in a space above the cavities, which makes the resonators mechanically weak.
Japanese Patent Laid-Open Publication No. 2002-140,075 (called the “Reference 3”) describes a piezoelectric thin film resonator in which a bottom electrode spans a cavity, and remains flat on the cavity. However, lateral dimensions are increased, and parasitic capacitance is increased between the bottom and top electrodes, which reduces an effective electric-mechanical coupling factor of the resonator. Further, Japanese Patent Laid-Open Publication No. Hei 9-130,200 (called the “Reference 4”) describes a piezoelectric thin film resonator in which a bottom electrode is processed in an area overlapping with a cavity by controlling residual stress of a thin film, so that no parasitic capacitance will be produced.
With the piezoelectric thin film resonator of the Reference 4, a lateral dimension of the lower electrode is smaller than that of the upper electrode, or both the bottom and top electrodes have the same size. The bottom and top electrodes are placed to face with each other, and a piezoelectric layer is inserted between the bottom and top electrodes, so that excited piezoelectric vibrations are propagated via the piezoelectric layer to a perimeter of the resonator. As a result, the Q performance will be worsened, or unnecessary spurious vibrations will be produced.